Hydrofluorocarbons are useful as etching gases for the microfabrication of semiconductors, liquid crystals, and the like. In particular, fluoromethane (CH3F) is drawing attention as an etching gas for forming state-of-the-art microstructures.
Known methods for producing fluoromethane are, for example, as follows:
(1) a method in which methyl alcohol and hydrogen fluoride are reacted using a catalyst (Patent Literature 1);
(2) a method in which methyl chloride and hydrogen fluoride are reacted using a catalyst (Patent Literature 2); and
(3) a method in which 1-methoxy-1,1,2,2-tetrafluoroethane is pyrolyzed (Patent Literature 3).
Among these methods, method (1) has drawbacks in that the catalyst tends to deteriorate because a large amount of water is produced, and corrosion tends to occur because hydrofluoric acid is produced from the unreacted hydrogen fluoride and the produced water. In method (2), it is necessary to add excess hydrogen fluoride to improve reactivity in fluorination. Recycling and reusing such hydrogen fluoride necessitates larger equipment, and thus the cost of production equipment becomes excessive. Further, there are problems with respect to decreased reactivity and corrosion due to moisture contamination or the like.
Method (3) requires energy for cooling to separate fluoromethane (boiling point of −79° C.) from difluoroacetyl fluoride simultaneously produced with fluoromethane, because difluoroacetyl fluoride has a low boiling point of 0° C. In addition, many impurities with low boiling points are contained, and separation of the impurities from fluoromethane is difficult even if rectification is performed. Among the impurities, in particular, trifluoromethane (CHF3), which has a boiling point of −84° C., is difficult to separate since the boiling point is close to that of fluoromethane. Besides, since the conversion of the starting material is associated with the amount of trifluoromethane produced, it may be necessary to decrease the conversion in the reaction to reduce the amount of trifluoromethane, thus posing the problem of a decrease in the production efficiency of fluoromethane. Furthermore, 1-methoxy-1,1,2,2-tetrafluoroethane used as a starting material is synthesized by reacting tetrafluoroethylene and methanol, and thus the method involves the risk of handling tetrafluoroethylene and causes a problem in that the costs of the starting material and equipment become expensive.
Meanwhile, cleaning gases, such as C2F6 and NF3, and etching gases, such as SF6, are dry process gases used in large quantities in production processes for semiconductors and liquid crystals. However, because these gases have large global warming effects, there is a demand for alternative gases. 3,3,3-trifluoro-2-(trifluoromethyl)propanoyl fluoride is expected to serve as such an alternative gas.
Patent Literature 4 listed below discloses that 3,3,3-trifluoro-2-(trifluoromethyl)propanoyl fluoride was obtained in a yield of 85% by cooling 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether to −40° C. to liquefy the ether, adding SbF5, and allowing a reaction to proceed at not greater than room temperature. However, this method is problematic in that the reaction is performed in a liquid phase using SbF5, which is expensive and corrodes metallic reaction kettles. Further, since the reaction is a batch reaction, the production efficiency is inferior to that of a continuous reaction in a gas phase. Thus, the method is unsuitable for industrial mass production. Patent Literature 4 also discloses that fluoromethane is simultaneously produced with 3,3,3-trifluoro-2-(trifluoromethyl)propanoyl fluoride by this method; however, Patent Literature 4 does not specifically disclose the production amount thereof, and the theoretical yield of fluoromethane is only 85% at most. Therefore, taken together with the above description that the yield of 3,3,3-trifluoro-2-(trifluoromethyl)propanoyl fluoride was 85%, the yield of 3,3,3-trifluoro-2-(trifluoromethyl)propanoyl fluoride and the yield of fluoromethane are both low. Accordingly, higher yields are desired.
Non-patent Literature 1 listed below discloses a method for producing 3,3,3-trifluoro-2-(trifluoromethyl)propanoyl fluoride, wherein 1,1,3,3,3-pentafluoro-2-trifluoromethylpropyl methyl ether is reacted with trimethylamine, followed by a reaction with hydrogen chloride. However, this method is inappropriate as an industrial production method, because the yield is only 46% and both alkali and acid are used in the reaction, thus making the reaction complex and requiring equipment with anticorrosion properties.